FAPC-193 Robert M. Kerr Food & Agricultural Products Center FOOD TECHNOLOGY FACT SHEET Adding Value to OKLAHOMA

405-744-6071 • www.fapc.biz • [email protected] Downstream Processing of Algal Cultures

Nurhan Dunford FAPC Oil/Oil Seed Specialist

Introduction Centrifugation of the culture results in efficient bio- In fact sheets (FAPC-191 and FAPC-192), principles mass recovery. Nonetheless, the capital and operating of photosynthetic microalgae growth and photobiore- costs of the centrifuges are high. Flocculation of the cells actor designs are discussed. This fact sheet will cover prior to filtration, centrifugation or settling is a common downstream processing of microalgae cultures, which practice to improve biomass recovery. Chemical addi- involves the following steps: 1) cell harvesting, 2) drying tives such as alum, lime, cellulose, salts, polyacrylamide and 3) biomass processing (i.e. extraction, conversion polymers, surfactants, chitosan and other synthetic fibers and refining of the crude product). have been examined as flocculants. Adjustment of the pH of the culture medium is an Cell Harvesting effective method for cell flocculation. An acid or base In general, biomass is separated from the growth solution is used for pH adjustment. Electroflocculation medium when microalgae cells reach the stationary phase and electrocoagulation methods, which are based on the of their growth. The highest biomass concentration in the manipulation of the electric field in the culture medium, culture medium would vary from 0.1 to 3 g/L (rarely up eliminate the need for chemical additives. Various com- to 5 g/L) depending on the reactor type, algae strain and binations of flocculation and other separation techniques the growth conditions. Microalgae harvesting is a very can be used to improve biomass recovery (i.e. floccula- challenging process because of the very small size of the tion + settling, flocculation + microfiltration or floccula- cells (1-30 micron) and dilute culture biomass concentra- tion + air flotation). tion, which requires handling of large volumes of water. The other harvesting technologies that have been Microfiltration and settling methods are not very examined for algal biomass recovery include: 1) grow- efficient for separating algal biomass from the culture ing microalgae on immobilized substrates, which can be medium because of the small size and low specific grav- easily removed from the culture medium, 2) ultrasound ity of the cells. Microfiltration is a simple and relatively induced aggregation followed by sedimentation, 3) bio- inexpensive process. The filter pore size is critically harvesting where microalgae are grown with higher or- important for efficient filtration. Small algae cells pass ganisms such as shrimp and fish and harvested together, through large pores resulting in biomass loss. When a and 4) bio-flocculation where algae are co-cultured with filtration medium with small pore size is used, cells get another organism that promotes sedimentation. For ex- trapped in the pores fouling the filtration medium; con- ample, an algae strain, Skeletonema, was used to form sequently, reducing the permeate flow rate and process flocs of Nannochloropsis, another microalgae strain. efficiency. Therefore, filter medium pore size needs to Microalgae harvesting costs can be substantial at 2-3 be optimized for specific applications. percent of the total system capital cost. The most efficient

Oklahoma Cooperative Extension Service • Division of Agricultural Sciences and Natural Resources flocculation technique and processing parameters for components. For instance, cellulose in algal biomass can a given operation will depend on the algae strain, cell be converted to intermediate platform chemicals. Then, concentration, pH and chemical composition of the me- these chemicals are reacted to produce a final product. dium. Cost, environmental impact and scalability of the This may require separation of cellulose, or the com- flocculation process, the effect of the residual flocculants pound of interest, from the other cell components prior in the harvested biomass on downstream processing, and to conversion. An alternative process could utilize whole quality of the water effluent for recycling and disposal cells for conversion. Utilization of whole cells rather than are very important factors that need to be evaluated while isolated compounds (extracted and purified) may reduce choosing a flocculation technique. the number of unit operations needed for a conversion process. Conversion efficiency may decline due to the Drying complex nature of the whole cell matrix, potential side Drying algal biomass may preserve its chemical reactions, and limited mass and energy transfer due to integrity, enhance shelf life, reduce shipping and trans- the presence of the other cell components in the reaction portation costs, and prevent microbial growth during medium. The end product, then, needs to be separated storage and handling, consequently, eliminating con- from the reaction mixture including the cell debris after tamination by other microorganisms. Furthermore, some the conversion. As such, biodiesel can be produced in situ of the downstream processes may require low moisture by using the whole cells rich in oil. Alternatively, oil is biomass (i.e. oil extraction). extracted from the cells using a solvent; cell debris and The traditional techniques such as spray drying, solvent are removed, and oil is refined before extracted freeze-drying, solar drying and convective hot air dry- oil is converted to biodiesel. ing have been used to dry algal biomass. Biomass is Chemical, biological or thermochemical pathways exposed to high heat for an extended time during drum can be utilized for converting algal biomass to fuels and drying to produce a product in flake form. Spray drying other products. Thermal processes use heat for the con- is efficient, but may rupture the cells during the high version. Torrefaction, pyrolysis and gasification are some pressure atomization of the culture and cause product of the thermal processes used for biomass conversion. degradation because of the high temperature in the dryer. Torrefaction of biomass, is a milder form of pyroly- Spray drying produces a product in powder form. It is sis that is performed at temperatures typically ranging difficult to maintain the quality of the biomass during between 200 and 320 degrees Celsius. Torrefaction leads open sun drying. Besides contamination issues, the slow to a dry product with no biological activity. During tor- drying rate due to low temperature may lead to biomass refaction, the biomass properties are changed to obtain degradation and microbial growth. A closed solar device a higher quality fuel for combustion and gasification generating a high temperature in the dryer could lead to applications. a high drying rate and may produce a good quality dry Pyrolysis is a thermochemical decomposition that biomass. Freeze drying minimizes biomass degrada- takes place at elevated temperatures (higher than 200– tion, but it is an expensive batch process. Selection of 300 degrees Celsius or 390–570 degrees Fahrenheit) in a suitable drying technique for a given application will the absence of oxygen or any halogen. The pyrolysis can depend on the type of the product to be produced from be carried out with or without a catalyst. Fast pyrolysis, the algal biomass. The cost of the drying process can be which is performed at about 500 degrees Celsius and 2-3 percent of the total system’s capital cost. very high heating and heat transfer rates, requires a finely ground dry biomass feed of typically less than 3 Biomass Processing mm particle size and short hot vapor residence times, Algal biomass can be used to produce a diverse less than 2 seconds, to minimize secondary reactions. range of products such as food, nutritional compounds, Bio-oil obtained by condensation of the pyrolysis gas omega-3 fatty acids, animal feed, organic fertilizers, needs to be refined to obtain the final product of interest. biodegradable plastics, recombinant proteins, pigments, Gasification also requires dry biomass and con- medicines, pharmaceuticals, vaccines and fuels including verts it into carbon monoxide, hydrogen and carbon jet fuel, aviation gas, biodiesel, gasoline and bioetha- dioxide at high temperatures (higher than 700 degrees nol. Conversion of biomass to an end product can be Celsius), without combustion in the presence of a con- achieved via selective conversion of individual biomass trolled amount of oxygen and/or steam. The resulting

193-2 gas mixture is called syngas or producer gas. Syngas content and then inoculated with the fermenting organism may be burned directly in gas engines, used to produce Saccharomyces cerevisiae. For this process enzymatic methanol and hydrogen, or converted into synthetic fuel hydrolysis is not required because the biomass is already or chemicals. Many of the conversion processes such as acid hydrolyzed in the previous step. Fermentation pro- syngas to methanol, olefins (ethylene and propylene), ceeds in batch mode for about 1.5 days and converts most and other similar chemical or fuel processes are based on of the sugars (primarily the hexose sugars glucose and the methods developed for coal (i.e. the Fischer-Tropsch mannose) to ethanol. The resulting dilute ethanol broth is synthesis). distilled to near azeotropic concentration and then puri- Hydrothermal liquefaction (HTL) is a thermal pro- fied to 99.5 percent. The slurry containing all residual cess that utilizes pressurized water to convert whole, wet solids can be used for lipid extraction using hexane. The algae to a liquid that can be further refined into fuels or micelle, crude oil + hexane, from the extraction process other products. First, algae culture is dewatered to about is routed to a stripping column to recover the solvent, 20 percent solid content (percent by weight). Then, the hexane. The desolventized crude oil needs to be refined liquefaction takes place in a reactor at high temperature to remove polar lipids and other impurities before con- and pressure. Four phases, which include oil, solid, aque- version to other products. The purified oil consisting ous and gas, are produced during the process. The oil can primarily of fatty acid based lipids can be hydrotreated be hydrotreated to produce diesel. The aqueous phase is to produce a product, which is suitable as a diesel blend catalytically treated to recover the carbon content and stock and a small amount of naphtha as a co-product. allow water to recycle back into the ponds. Process off The oil can also be converted to biodiesel by chemical gas may be used to generate hydrogen, heat and/or power. or enzymatic transesterification reaction. Biochemical conversion processes make use of the Carbon dioxide generated during the fermentation enzymes of bacteria and other microorganisms to break process can be recycled to upstream algal cultivation down biomass. In most cases, microorganisms are also ponds. The aqueous phase from the entire process can used to perform the conversion process: a) anaerobic be routed to anaerobic digestion for biogas production digestion, b) fermentation and c) composting. In general, and/or recycled to the process after treatment. The di- biochemical processes are carried out under mild condi- gester’s effluent water and solid cake contain nitrogen tions that may minimize the formation of by-products and phosphorus nutrients that can be used as fertilizer. and preserve other potentially valuable co-products such as proteins, carotenoids and vitamins. Biochemical Conclusions conversion processes may potentially eliminate some The systems for algal biomass production as aquacul- of the algal biomass pretreatment steps (i.e. drying and ture feed have been around since the 1950s. In the U.S. extraction). there are several companies marketing microalgae based Biodiesel production can be performed by simultane- dietary supplements and high-value specialty products ous enzymatic hydrolysis, esterification and transesteri- such as oils. Most of the microalgae to biofuel, bioprod- fication of whole algae or algal oil extracts. Application uct or chemical production processes discussed in this of multiple enzyme cocktails to whole algae enables fact sheet are still at developmental stage. The process simultaneous or sequential production of lipid and fer- economics rather than the technological issues appear to mentable sugar-based products. be the main hurdle for commercial utilization of algal An algal biomass conversion process, which is still biomass as feedstock for diverse range of products to under investigation, includes the following steps: a) replace fossil fuels or nonrenewable feedstock. biomass pretreatment, b) bioconversion and c) refin- ing. First, algal biomass (20 percent solids by weight) is combined with steam and treated with dilute sulfuric acid catalyst at a high temperature for a short time to hydrolyze the glucan carbohydrates to monomeric sugars and make the remaining biomass more amenable to lipid extraction. The pH of the treated slurry is raised to about 5 by adding ammonia prior to fermentation. The treated biomass is cooled and brought back to 20 percent solid

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